As civil engineering and construction machines, e.g., hydraulic excavators, there is known the type of machine that includes a plurality of actuators such as a boom cylinder, an arm cylinder, a bucket cylinder, a travel motor and a swing motor, and a hydraulic drive system for supplying a hydraulic fluid to those actuators includes a plurality of variable restrictors in communication with a delivery line of a hydraulic pump, the variable restrictors being operated to supply the hydraulic fluid to the corresponding actuators. However, without operation of such a hydraulic drive system there is a fear that when plural variable restrictors are operated simultaneously, the hydraulic fluid is supplied to the actuator having a small load pressure, but not supplied to the actuator having a large load pressure.
For that reason, there has been proposed a hydraulic drive system as disclosed in JP, A, 2-248705. This prior-art system comprises first and second variable displacement hydraulic pumps, a first hydraulic actuator driven by hydraulic fluids delivered from the first and second hydraulic pumps, a second hydraulic actuator driven by the hydraulic fluid delivered from the second hydraulic pump, a first variable restrictor for controlling a flow rate of the hydraulic fluid supplied from the first hydraulic pump to the first hydraulic actuator, a second variable restrictor for controlling a flow rate of the hydraulic fluid supplied from the second hydraulic pump to the first hydraulic actuator, a third variable restrictor for controlling a flow rate of the hydraulic fluid supplied from the second hydraulic pump to the second hydraulic actuator, a first pressure compensating device for controlling a differential pressure across the first variable restrictor, a second pressure compensating device for controlling a differential pressure across the second variable restrictor, a third pressure compensating device for controlling a differential pressure across the third variable restrictor, a first regulator for controlling a delivery rate of the first hydraulic pump, a second regulator for controlling a delivery rate of the second hydraulic pump, and a coupling circuit for joining the flow rate passing through the first variable restrictor and the flow rate passing through the second variable restrictor with each other and supplying the joined flow rate to the first hydraulic actuator.
The above prior-art system further comprises a first check valve for detecting a pressure on the outlet side of the first pressure compensating device, a second check valve for detecting a pressure on the outlet side of the second pressure compensating device, a third check valve for detecting a pressure on the outlet side of the third pressure compensating device, and a signal pressure supply circuit for selecting maximum one of the pressure detected by the first check valve, the pressure detected by the second check valve and the pressure detected by the third check valve, and supplying the selected maximum pressure as a common signal pressure to the first and second regulators.
The first and second variable restrictors, the first and second pressure compensating devices, and the first and second check valves constitute one valve apparatus. Also, the first and second variable restrictors are formed in a common slidable spool.
In the case of solely driving the first actuator in the prior-art system thus constructed, when the spool is moved in one direction through a predetermined distance, the first variable restrictor is opened and the hydraulic fluid from the first hydraulic pump is supplied to the first actuator via the first variable restrictor and the first pressure compensating device, thereby driving the first actuator. When the spool is further moved under such a condition, the second variable restrictor is opened and the hydraulic fluid from the second hydraulic pump is delivered via the second variable restrictor and the second pressure compensating device to join with the hydraulic fluid from the first hydraulic pump before being supplied to the first actuator, thereby driving the first actuator at an increased speed.
In the case of solely driving the second actuator, the third variable restrictor is opened and the hydraulic fluid from the second hydraulic pump is supplied to the second actuator via the second variable restrictor and the second pressure compensating device, thereby driving the second actuator.
In the case of simultaneously driving the first and second actuators, the first to third pressure compensating devices control pressures downstream of the first to third variable restrictors, i.e., pressures between the first to third variable restrictors and the first to third pressure compensating devices, to become equal to the maximum load pressure selected by the first to third check valves and the signal pressure supply circuit. Accordingly, the hydraulic fluid from the first hydraulic pump can be properly distributed and supplied to the first and second actuators regardless of a difference in the magnitude of load pressure between the first and second actuators, thereby enabling combined operation of both the actuators.